A hydraulic excavator as one type of construction machinery generally includes a prime mover such as an engine, a hydraulic pump driven by the prime mover, hydraulic actuators including hydraulic cylinders for driving, for example, a boom, an arm, a bucket, and a swing structure using hydraulic oil delivered from the hydraulic pump, and a control valve (operating valve) that supplies the hydraulic oil from the hydraulic pump selectively to the hydraulic actuator. In such construction machinery, in order to reduce driving power of a driving power source and fuel consumption of the entire construction machinery, a known technique recovers potential energy of the boom that falls by its own weight and inertia kinetic energy of the swing structure to achieve effective use of these types of energy.
One known arrangement, for example, includes a recovery device that recovers return oil from a hydraulic actuator. In the arrangement, after the recovery of the return oil, a regenerative device regenerates a flow rate and, when the regenerative flow rate is to be merged with a discharge flow rate from a hydraulic pump, the discharge flow rate from the hydraulic pump driven by a driving device, such as an engine, is varied according to the regenerative flow rate (see, for example, patent document 1).